1. Field of Invention
This invention relates generally to semiconductor light emitting devices and more particularly to active regions for III-nitride light emitting devices.
2. Description of Related Art
Semiconductor light-emitting devices such as light-emitting diodes (LEDs) are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness LEDs capable of operation across the visible spectrum are Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride layers are epitaxially grown on sapphire, silicon carbide, or gallium nitride substrates.
III-nitride devices are usually grown with a light-emitting or active region that includes multiple quantum well layers separated by barrier layers. The active region is sandwiched between a p-type region and an n-type region. The p- and n-type regions supply positive and negative charge carriers (electrons and holes) to the quantum wells in the active region, where the positive and negative charge carriers recombine to generate light. The brightness of a light emitting device is at least partially determined by the internal quantum efficiency of the device, which is an indication of how many light-emitting electron-hole recombinations occur in the active region.
Each quantum well layer can hold a finite number of charge carriers at a given time. The carrier capacity of a semiconductor layer depends on how much material is present in the layer, thus the thicker a quantum well layer, the more carriers that quantum well layer can hold. However, in III-nitride devices, the quantum well layers are typically InGaN, which has a poor crystal quality compared to other III-nitride layers due to the large size of indium atoms and the amount of indium required to make the quantum well layer light emitting. Another complication is that InGaN is usually grown at a lower temperature than GaN, which results in degraded crystal quality. Additionally, there is In fluctuation in the InGaN layers that limits the carrier capacity of the light emitting layers. Piezoelectric fields can cause decreased overlap of electron and hole wavefunctions, with decreased oscillator strength for recombination. Finally, defects in the crystalline semiconductor layers of a device can cause non-radiative recombination of positive and negative charge carriers, which can reduce the amount of light generated by a device by robbing the quantum well layers of charge carriers. Thus, in order to maintain the crystal quality and internal quantum efficiency of the light emitting device, the quantum well layers are generally thin and the barrier layers separating the quantum well layers are generally thick layers of better crystal quality than the In-containing quantum well layers.